The long-range goal of this research is to elucidate the role of cyclic nucleotide-and Ca ions-stimulated protein phosphorylation in the physiological and pharmacological regulation of smooth muscle contraction. There is evidence that many of the responses of neurotransmitters and hormones are mediated by phosphorylation of key proteins. In our studies, we propose to define the role of Ca 2 ion and cyclic nucleotide-sensitive proteins in the regulation of smooth muscle contraction. Our approach will allow a comparison of specific proteins phosphorylated in intact tissue with those phosphorylated in isolated fractions. We will carefully examine the relationship between the degree of phosphorylation of these proteins and: (1) the amount of contraction and relaxation of rat myometrium; (2) Ca ions sequestration properties of relatively pure fractions of myometrial sarcolemma. The proposed investigation is important for (1) resolving the current controversy on the existence of effective cellular compartments for cyclic nucleotides in smooth muscle; (2) elucidating the role of the Ca ions-stimulated phosphorylation in smooth muscle contraction; (3) providing fundamental information on the mechanisms by which neurotransmitters, hormones, and therapeutic agents may regulate smooth muscle tension.